Monkeys in Malaysian forests are a reservoir for a rare form of malaria that could become a significant cause of disease in humans throughout South-East Asia, a study warns.

The malaria parasite Plasmodium knowlesi is transmitted between monkeys by forest-dwelling mosquitoes. This limits transmission to humans and, at present, there are only around 300 human cases per year.

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But as human populations grow and the forest shrinks, people are likely to venture into the forest more often and in greater numbers. This, said the researchers, could lead the parasite to evolve, enabling it to pass more easily between humans and monkeys.

Balbir Singh, director of the Malaria Research Centre at the University of Malaysia, Sarawak (UNIMAS), and colleagues first showed in 2004 that P. knowlesi causes deadly disease in humans. They did not know whether the disease is maintained in the human population or whether the monkeys act as wild reservoirs from which humans get infected.

Singh, who led the current study, published last week (7 April) in PLoS Pathogens, said he suspected macaque populations in Sarawak, Borneo, were a reservoir for P. knowlesi — and were the source of human malaria cases seen in local hospitals — as monkey populations in other areas are also known to act as malaria reservoirs.